Large electrical generators require external excitation which is provided by the exciter. Conventional systems provide exciter output connected to the generator field through a series of slip rings or brushes. An alternative system is a brushless exciter which is joined directly to the generator field winding without the need for brushes or slip rings to carry power. Brushless excitation systems employ an internal diode bridge to rectify the ac current generated by the rotating armature.
In the event of a diode failure in the internal rectification system, rapid protection is required. This is particularly true of high power density brushless exciters as such systems have low thermal capacity. A failed diode can cause a phase-to-phase fault which can result in destructively high winding currents. Typically, the armature winding itself is not accessible for instrumentation required for testing or monitoring. As a result, it has been previously known to obtain information from the magnetic field windings by monitoring transformer action. This has been shown, for example, in U.S. Pat. Nos. 4,106,069 and 4,528,493.
In more recent machine designs, however, this type of monitoring is not effective due to the presence of strong damper systems which act to isolate the magnetic field of the winding. The damper systems further add excessive attenuation and delay to any signal which could be obtained from the field winding. These problems are even accentuated in cryogenic machines. Cryogenic machines exhibit high power density made possible by efficient cooling systems and by the use of cryogenic conductors. The cryogenic conductors allow high current densities with minimal ohmic loss due to the extremely low resistivity of the materials at cryogenic temperatures which may be on the order of twenty degrees Kelvin. One drawback of such machines is their low thermal capacity. Electrical faults in the generator, as can be induced by a failed diode, result in fault currents which may be 5 to 10 times rated current. Such high currents rapidly lead to higher conductor temperatures which can quickly become destructive to the entire system.
In order to reduce the probability of faults due to failed diodes, it has been known to provide a redundant series diode. However, there is still a possibility that even a second diode can fail. Moreover, in low loss machines such as those discussed herein, a second diode can present a substantial increase in loss because the excitation loss is essentially doubled due to the second diode. Thus, the need for additional protection still exists.
Another difficulty presented particularly with the cryogenic machines is that of delay in detection of a fault. As stated, conventional methods of protection involve monitoring the field winding for a change in the signal indicating a change in ac currents in the armature. However, if the ac signal induced in the field winding is strong, many cycles would have passed after the fault occurred because a time delay would occur while the magnetic field diffuses through the damper system and into the field winding. It would only be after the change occurred in the signal from the field winding that the fault would be detected. In the low thermal capacity machine, severe heating could have occurred in the armature winding during the delay.
There remains a need for a device which can accurately detect faults in a brushless excitation system without appreciable delay. There remains a further need for a protection mechanism which does not depend upon obtaining a measurement from the exciter field winding. There is also a need for a protection device which is operable and effective in the cryogenic class of machines.